The present invention relates to a knee joint endoprosthesis having a femur element and a tibia element.
Such systems are known, for example, from DE-A 41 41 757. There, the femur element, viewed from medial to lateral, has an essentially U-shape, wherein one horizontal, two diagonal and two vertical joint surfaces pointing toward the femur are provided for installation on a resected femur bone. The femur element is provided with two runners which reproduce the natural condyles and condyle rollers.
The tibia element of known systems usually has two sliding tracks, on which the runners of the femur element can execute a rolling away motion and, optionally (depending upon the type of endoprosthesis), sliding movements. The tibia element has a horizontal support surface pointing toward the tibia for supporting the resected tibia.
The tibia element as well as the femur element of known systems respectively have a conical pin, which is insertable into a conical taper socket in a respective shank element of the modular system. Between pin and socket a conical friction (clamping) connection can be produced, which should ensure a durable connection between femur element and femur shank element or tibia element and tibia shank element.
The shanks of modular systems are implanted either by means of a bone cement, for example PMMA, into the marrow space of the respective tubular bone, or without cement, whereby the shank element is then provided with a special surface into which and through which bone material can grow for durable secondary fixation of the implant (see, for example, German Patent DE-C-195 435 30).
The following problems are associated with the known systems:
The shanks (generally made of metal), which are installed in the marrow space of the respective tubular bone, have a completely different elasticity modulus than the bone material (spongiosa) surrounding the shank. This often leads (especially with older persons, whose bones are diseased with osteoporosis) to fatigue fractures above the shank end in the marrow space precisely because of the different elasticity moduli.
The second problem concerns the natural valgus angle, which lies in the range of 7.5.degree. with most people. With the artificial endoprosthesis systems, however, only valgus angles of at most 5.degree. have been producible up until now, and to be sure owing to the previously mentioned conical clamping usually used between the shank element and the femur element or tibia element, since the desired conical clamping no longer occurs with an angle &gt;5.degree..
Moreover, finding sufficient hold for a new shank endoprosthesis, so that this can be implanted sufficiently firmly, is problematic with the known shank endoprostheses in the event that a revision intervention should become necessary. This is especially critical in younger patients since, according to previous experience, implanted endoprostheses have a limited lifetime of 10 to 15 years, assuming that an unforeseen revision intervention is not necessary before this. With younger patients, it is then in any case necessary to replace the originally implanted endoprosthesis with a new one. This is not always possible, without further measures, for the reasons mentioned.